1. Field of the Invention
The present invention is directed generally to cryogenic refrigerators and more specifically to adsorption type refrigeration systems in which a low pressure refrigeration fluid is adsorbed by the surface of a sorbent material and then pressurized by heating the sorbent material.
2. Description of the Prior Art
Adsorption type refrigerators can pressurize a refrigeration fluid such as methane gas (CH.sub.4) without using a reciprocating piston or other moving compression means. Because fewer moving parts are required for such refrigeration systems, the systems exhibit relatively long lifetimes (e.g. ten years or more) and usually operate with little or no mechanical vibration. The lack of mechanical vibration is particulary advantageous when sensitive transducers need to be cooled without being disturbed by vibrations.
The operation of an adsorption type refrigerator can be summarized as follows. A refrigeration fluid is precooled and adsorbed onto the cavity walls of a porous adsorption material, at a first temperature. Charcoal is often used as the adsorption material. The sorbent (charcoal) and sorbate (methane) are then heated to a higher second temperature while the sorbate is trapped in the sorbent. The pressure of the trapped sorbate rises with the temperature as the sorbate is driven off the sorbent surface. An outlet valve is opened to direct the pressurized sorbate away from the sorbent to the high pressure side of a high pressure/low pressure orifice. Heat is absorbed (and cooling takes place) when the pressurized sorbate (refrigeration fluid) expands into the low pressure side of the orifice.
Methane gas has been the material of choice for cooling to temperatures in the range of approximately 140 degrees Kelvin (K). Methane is selected because it can be preliquefied at a temperature above 140 K but below its critical temperature of 190.7 K. Refrigeration efficiency is believed to be enhanced by preliquefying the refrigeration fluid before passing it through an expansion orifice. The use of methane poses a number of problems however, including: (a) the danger of an explosion occurring if the methane is accidentally exposed to oxygen or other reactive chemicals; (b) an undesirable tendancy of methane to slowly decompose and/or react with other materials to thereby deplete the system of its working fluid while also contaminating the surfaces of the absorption material with elemental carbon and hydrogen or blocking pores in the sorbent material with large molecules; and (c) the association of methane with refrigeration efficiencies that are relatively poor in comparison to the efficiencies of other refrigeration fluids. Refrigerants such as Freon 14, ethylene, and nitrous oxide have been found to pose some if not all of these problems to a certain degree.